Interest in natural convection in vertical channels has significantly increased over the past few years because of its application in solar energy systems. One of the main challenges regarding these systems is to understand the intimate and complex interactions between the natural convective flow and the surrounding conditions. The problem that is considered here can be explored in the study of a spatially developing transitional natural convection flow subjected to various stresses from the external environment with a focus on the external thermal stratification which is inherent to experimental measurements. In this presentation, Spectral Proper Orthogonal Decomposition is used to study the complex structures that develop in this transitional flow. Then, the role of these structures in the transition process as well as the definition of different indicators of the transition will be tackled. Finally, the effect of the external thermal stratification on the transitional flow is analysed.